The invention relates to a process for the production of water-expandable styrene polymers (WEPS) by polymerization of styrene in aqueous suspension, the suspended styrene droplets comprising an emulsion of finely divided water.
Particulate expandable styrene polymers (EPS) are normally prepared by polymerization of styrene in aqueous suspension in the presence of a volatile organic expanding agent. Commonly used expanding agents are hydrocarbons, particularly pentane. For environmental reasons, pentane emitted during production and processing of EPS must be re-collected. This is an elaborate and cost-intensive procedure. Thus it is expedient to replace these organic substances in the long run by more acceptable expanding agents, for example water.
A dissertation of the University of Eindhoven xe2x80x9cWater Expandable Polystyrenexe2x80x9d by J. J. Crevecoeur dating from 1997 describes a process for the production of WEPS, in which water, finely distributed in styrene, is first of all emulsified by means of surface-active substances, after which the styrene is polymerized up to a conversion of 50%, the mixture is suspended in water with phase inversion and the styrene is finally polymerized to completion by means of peroxide initiators. The surface-active substances used are amphiphilic emulsifiers, eg sodium bis(2-ethylhexyl)sulfosuccinate or sodium styrenesulfonate or block copolymers comprising polystyrene blocks and poly(styrene sulfonate) blocks. All of these substances exhibit both a hydrophilic and a hydrophobic radical and are thus capable of emulsifying water in styrene. Further two-stage processes are described in WO 98/01489 and WO 01,501.
According to Patent Applications WO 99/48936, WO 99/48956 and WO 98/48957 WEPS is prepared directly in a one-stage process in aqueous suspension, use being made of thermolabile organic compounds, finely divided inorganic solid materials or EPS recyclates as emulsifying auxiliaries.
In all of said processes WEPS particles are formed which contain water as the sole expanding agent. However, these cannot be pre-expanded by conventional EPS foaming processes using steam, but instead, use must be made of air having a temperature above 100xc2x0 C. or hot oil baths or hot silicone oil baths.
It is thus an object of the present invention to provide polystyrene particles containing water as expanding agent, which can be expanded at conventional prefoaming temperatures using ordinary steam.
This object is achieved by the process of the invention, in which the suspending agent is a mixture of water and an organic liquid, which is water-miscible and which is virtually a non-solvent for polystyrene and exhibits a boiling point below 100xc2x0 C.
The suspension polymerization of the invention is preferably carried out in a single stage without previous mass polymerization. Known emulsifying auxiliaries can be used which cause water to be emulsified in fine distribution in the styrene droplets, eg:
a) water- and styrene-soluble inorganic solid materials having an average particle size of not more than 20 xcexcm and a density of more than 1.1 g/cm3 in an amount of from 0.1 to 15 wt %, based on the monomers, as described in WO 99/48957. Examples of suitable compounds are silica gel, silicates, metal oxides, metal hydroxides and metal salts, preference being given to from 0.2 to 5 wt % of carbon black having an average particle size of from 10 to 500 Nm and from 0.4 to 10 wt % of graphite having an average particle size (longest flake diameter) of from 1 to 30 xcexcm and preferably from 2 to 20 xcexcm.
b) EPS recyclate, which from its production stage still contains conventional amphiphilic coating materials, according to WO 99/48956. The EPS recyclate is added in an amount of from 5 to 30 wt %, based on the monomers, as a styrenic solution of the suspension. EPS recyclate generally contains from 0.1 to 2 wt % of protective material, eg antistatic agents, such as quaternary ammonium alkylsulfonates, aliphatic sulfonates or alkoxylated ammonium salts; antitack agents, such as metal salts of fatty acids, eg zink stearate, further fatty acid esters and fatty acid amides; agents for reducing the cooling time, such as glycerin esters and hydrophobic esters of fatty acids or fatty alcohols, eg glycerin monostearate or tristearyl citrate.
c) thermolabile organic compounds which are soluble or dispersible in styrene and which are completely or partially decomposed or hydrolyzed to amphiphilic and/or hydrophilic compounds at elevated temperature, according to WO 99/48936. The temperature of decomposition should be above 80xc2x0 C. but below the maximum polymerization temperature. They are used in an amount of from 0.1 to 5 wt %, based on the monomers. The following are suitable: halo compounds, such as hexabromocyclododecane having a temperature of decomposition of 125xc2x0 C. (in the polymerization medium) further 1,1,2,2-tetrabromoethane having a temperature of decomposition of 128xc2x0 C. or chlorinated paraffins having a temperature of decomposition of approximately 130xc2x0 C. During decomposition or hydrolysis of these halo compounds there are formed amphiphilic hydrocarbons carrying a hydrophilic hydroxyl group, and, in addition, hydrophilic hydrogen halides. Also suitable are peroxides, such as dibenzoyl peroxide having a temperature of decomposition of 80xc2x0 C., tert-butylperoxy-2-ethyl hexanoate having a temperature of decomposition of 80xc2x0 C., and dicumyl peroxide having a temperature of decomposition of 110xc2x0 C. During decomposition of these peroxides there are again formed amphiphilic hydrocarbons carrying carboxyl groups or hydroxyl groups. Also suitable are organic phosphorus compounds, such as aryl phosphates and derivatives thereof.
d) amphiphilic organic emulsifiers carrying both hydrophilic and hydrophobic groups, according to WO 98/01489, preferably in amounts of from 0.1 to 5 wt %, based on the monomers. Preference is given to bisalkylsulfosuccinates, sorbitol (C8-C20) carboxylates and C8-C20 alkylxylene sulfonates.
e) polar group-containing, water-soluble polymers as described in WO 98/01501, preferably in amounts of from 2 to 20 wt %, based on the monomers. Preference is given to polyvinylpyrrolidone, whilst starch, esterified starch, polyvinyl alcohol, polyvinyl acetate, polyacrylic acid, polyethylene glycols and cellulose derivatives are also suitable.
An alternative is a two-stage process in which the monomers are first of all prepolymerized in substance to a conversion of from 15 to 70% and preferably from 25 to 50% and the prepolymer, containing an emulsifying auxiliary, is suspended in the suspending agent, where it is polymerized to completion. This two-stage process is described in detail in WO 98/01489 and WO 98/01501. The emulsifying auxiliary can be present during the prepolymerization or it may be added following termination thereof. Use can be made of the aforementioned emulsifiers, preferably those mentioned under d) and e). Prepolymerization is carried out under conventional mass polymerization conditions, preferably at from 80xc2x0 to 120xc2x0 C. in the presence of free radical initiators. After achieving a conversion of from 15 to 70% and preferably from 25 to 50% the viscous prepolymerization solution is suspended in the suspending agent. The water which will subsequently act as expanding agent may be added during prepolymerization.
In both cases polymerization of the invention is completed in suspension, the suspending agent being a mixture of water and an organic liquid, preferably in a ratio, by weight, of from 99:1 to 50:50 and more preferably from 95:5 to 75:25. The organic liquid should be water-miscible, and polystyrene should be virtually or, preferably, absolutely insoluble therein. Its boiling point should be below 100xc2x0 C. and in particular between 40xc2x0 to 80xc2x0 C. Suitable organic liquids are C1-C3 alcohols, C3-C5 ketones and C1-C5 aldehydes. Particular preference is given to ethanol. Furthermore, the suspending agent can contain C4-C7 alkanes, particularly pentane, preferably in amounts of up to 3 wt %, based on the monomers.
In the suspension polymerization of the invention the monomer used is preferably styrene alone. However, up to 20% of its weight can be replaced by other ethylenically unsaturated monomers, such as alkyl styrenes, divinyl benzene, acrylonitrile, 1,1-diphenylethylene or xcex1-methylstyrene.
When carrying out the suspension polymerization, conventional auxiliaries, such as suspension stabilizers, free radical initiators, flameproofing agents, chain-transfer agents, expanding agents, nucleation agents and plasticizers, can be added. It is advantageous to use, as suspension stabilizers, inorganic Pickering dispersing agents, eg magnesium diphosphate or tricalcium orthophosphate, together with small amounts of alkyl sulfonates. Preferred flameproofing agents are organic bromine compounds, such as hexabromocyclododecane, which are added in amounts of from 0.1 to 2 wt %, based on the monomers.
It is advantageous to carry out polymerization in the presence of from 1 to 30 wt % and preferably from 3 to 15wt % of polystyrene, which is advantageously used in the form of a styrenic solution. Instead of pure polystyrene, polystyrene recyclate may alternatively be used.
The emulsifying auxiliary is preferably added at the very start of the suspension polymerization; alternatively, it can be metered in during polymerization until a conversion of 90% has been reached.
The suspension polymerization is advantageously carried out at two temperature levels, use being made of two peroxide initiators which decompose at different temperatures. The suspension is first of all heated to 80xc2x0 to 90xc2x0 C., in which case the first peroxide, eg dibenzoyl peroxide, decomposes and polymerization starts. The temperature is then allowed to slowly rise to from 100xc2x0 to 140xc2x0 C. During this operation, the second peroxide, eg dicumyl peroxide or di-tert-butyl perbenzoate decomposes.
During the suspension polymerization the batch should be stirred vigorously. For this reason and due to the assistance of the emulsifying auxiliary, water is emulsified in fine distribution in the suspended styrene droplets or prepolymer droplets.
The expandable styrene polymers manufactured by the process of the invention contain, as expanding agents, from 2 to 20 wt % and preferably from 5 to 15 wt % of water and from 0.1 to 10 wt % and preferably from 0.2 to 10 wt % of the organic liquid, and optionally up to 3 wt % and in particular from 0.2 to 2.5 wt % of a C4-C7 alkane, preferably pentane. The organic liquid lowers the boiling point of the expanding agent and has the effect that the WEPS particles can be expanded with steam having a temperature of from 100xc2x0 to 110xc2x0 C. The polystyrene-miscible alkane lowers the softening point of the styrene and thus also assists in decreasing the foaming temperature.
The particle size of the WEPS particles is from 0.2 to 5 mm and preferably from 0.5 to 2 mm. They can be expanded to foamed particles with steam in conventional pre-expanders, and expansion can be carried out once or a number of times.
The expanded particles of WEPS can, like conventional expanded particles of EPS, be welded to form foamed panels, foamed blocks or foamed shaped articles having densities of from 10 to 100 g/L, which can be used as insulating or packaging materials.
In the following example, the percentages are by weight.